260 ZOOLOGY 
The characteristic larval stage of the Entomostraca, the 
Nauplius, is not present in the Cladocera, except in the genus 
Leptodora, and there it is confined to the winter-egg genera- 
tion. 
The Cladocera are as a rule freshwater inhabitants, living 
in enormous numbers in lakes, ponds, and springs; only a few 
species flourish in brackish water or in the sea. 
b. Branchiopoda.—The branchiopoda are Phyllopods of 
considerable size, with clearly segmented bodies often partially 
covered by a shield-shaped or laterally compressed shell. They 
possess from ten to sixty pairs of foliaceous swimming append- 
ages, which bear well-developed gulls. 
The Branchiopoda differ from the Cladocera in the fact 
that they are larger, consist of more segments, and are generally 
more complicated in their structure. They present considerable 
differences in the various species. Lstheria and Limnadia bear 
large bivalved shells which completely enclose the body, Apus 
has a shield-shaped dorsal shell which covers the head and 
body, but leaves the tail free, whilst Branchipus is devoid of 
any shell. 
The flattened leaf-like appendages of Apus have been 
looked upon as a primitive type from which the appendages of 
other Crustacea may be derived. This type of appendage is 
almost completely retained in the foliaceous maxillae of Astacus. 
The abdominal appendages, that is, those which are situated 
behind the genital openings, are the least specialised, and these 
present an unjointed axis which bears on its inner edge six 
processes termed endites, which bear numerous setae. The 
axis ends in a sub-apical lobe, and carries on its outer sides 
two exites; of these the distal is large, and has been termed 
the flabellum, the proximal is devoid of setae, and forms the 
branchia. 
In the female the appendage of the 11th thoracic segment, 
which bears the opening of the oviduct, is modified; the 6th 
endite and the sub-apical lobe are enlarged to form a hollow 
cup, over which the flabellum closes like a lid, this forms a 
receptacle for the ova, and the appendage is known as the 
oostegopod. 
